The present invention generally relates to semiconductor chip assembly, and more specifically, to optimized solder pads for flip chip bonding.
Solder pads can be used in the micro-electronics industry to make electrical and mechanical contacts or connections between integrated circuits (ICs) or between printed circuit boards (PCBs). Solder pads can also be used to make electrical and mechanical contacts or connections between opto-electronic and integrated photonic circuits and components.
To achieve such electrical and mechanical contacts and connections, the surfaces of two or more circuits to be connected can be coated with metal pads formed of one or more thin metal films, also and called Under-Bump Metallization pads (UBM pads) or solder pads. A bottom circuit of a two circuit assembly can be the larger of the two circuits and is also referred to herein as a substrate. A top circuit of a two circuit assembly can be the smaller of the two circuits and is also referred to herein as a chip.
Solder material, such as tin, indium, bismuth, or any combination of tin, silver, copper, gold, bismuth, indium or lead, can then be deposited on some of the metal pads on at least one of the circuits. This is typically accomplished through an electroplating process where the circuit is immersed in an electroplating bath during solder deposition. Optionally, solder paste printing, preform solder ball drop, or solder jetting method can be applied for solder deposition. Thereafter, the surfaces of two circuits to be connected can be brought into close contact and then temperature can be temporarily elevated beyond the melting temperature of the solder metal. When the solder metals are heated beyond the melting temperature, the melted solder can wick adjacent UBM pads and establish electrical contacts between the two circuits in close contact. Upon cooling, the solder can solidify to mechanically and electrically connect the two circuits.
The amount of solder deposited on UBM pads can impact the behavior and integrity of contacts between two circuits. Many applications using UBM pads and solder involve a very large number of potential solder connections and have a relatively small area on the chip between the solder connections. Adjusting the amount and uniformity of the solder on a large array of UBM pads can be a challenging task. If too little solder is used at a particular location, for example, an electrical connection can be incomplete or fragile to mechanical stress and fatigue, potentially resulting in system failure due to an electrical open circuit. On the other hand, if too much solder is used at a particular location, additional solder could result in bulges in the solder bumps and spurious contacts between adjacent solder bumps, potentially resulting in a failure due to an electrical shorting. Industry trends favoring ever smaller microelectronic components call for further reductions in space between solder contacts, while uniformity of solder plating can be on the order of 10% when wafers of 200 or 300 mm in diameter are plated. This can increase the potential of defects or degradation of the contacts and system failure.
Soldering applications can also be used during assembly of opto-electronic assemblies. Some applications call for alignment of chips in such assemblies on the order of micron or sub-micron precision. For example, semiconductor lasers generate infra-red radiation in a tightly confined channel, or waveguide, wherein the size of the beam emanating from the laser can be on a scale of one half a micron to 5 microns. In such applications, horizontal and vertical forces generated during solder melting of chip assembly can be used to assist with chip alignment. However, such applications can be highly sensitive to over-plating and under-plating of solder resulting from the electroplating process.